1. Field of the Invention:
This invention relates to the art of papermaking, particularly to treating paper products with milk and then high temperature to improve its properties, including wet stiffness, wet tensile strength and bursting strength.
2. Description of the Prior Art:
There is currently considerable interest in improving various properties of paper and boards. Quantifiable paper properties include: dry and wet tensile strength, folding endurance, stiffness, compressive strength, and bursting strength, among others. Which qualities should desirably be enhanced depends upon the intended application of the product. In the case of milk carton board, for example, stiffness is of particular importance, whereas for linerboard, wet strength, folding endurance, and high humidity compression strength may be more important.
All of these properties can be measured by well-known standard tests. As used herein, then, "wet strength" means wet tensile strength as measured by American Society for Testing and Materials (ASTM) Standard D829-48. "Folding endurance" is defined as the number of times a board can be folded in two directions without breaking, under conditions specified in Standard D2176-69. "Stiffness" is defined as flexural rigidity and is determined by the bending moment in g-cm. "Linerboard", as used herein, is a medium-weight paper product used as the facing material in corrugated carton construction, and is usually made from pulp produced by the kraft process. Folding carton board is a medium to heavy weight paper product made of unbleached and/or bleached pulps having basis weights from 40-350 g/m.sup.2.
Prior workers in this field have recognized that high-temperature treatment of paperboard can improve its wet strength. See, for example E. Back, "Wet stiffness by heat treatment of the running web", Pulp & Paper Canada, vol. 77, No. 12, pp. 97-106 (Dec. 1976). This increase has been attributed to the development and cross-linking of naturally occurring lignins and other polymers, which phenomenon may be sufficient to preserve product wet strength even where conventional synthetic resins or other binders are entirely omitted.
It is noteworthy that wet strength improvement by heat curing has previously been thought attainable only at the price of increased brittleness (i.e., reduced folding endurance). Embrittled board is not acceptable for many applications involving subsequent deformation, and therefore heat treatment alone, to develop the wet strength of paperboard and carton board, has not gained widespread acceptance. Heat treatment has most successfully been used to produce hardboard. It has not been practiced on paper having latex or milk additives.
It is therefore an object of the invention to produce paperboard having both improved stiffness and wet strength, and adequate folding endurance.
With a view to the foregoing, a process has been developed which dramatically and unexpectedly increases not only the stiffness and wet tensile and bursting strengths of various paperboards, but also preserves their folding endurance. In its broadest sense, the invention comprises steps of (1) applying a natural latex, preferably milk, to paperboard, and then (2) heating the paperboard so treated to an internal temperature of at least 400.degree. F. (205.degree. C.) for a period of time sufficient to increase the wet strength of the board. We prefer to raise the internal temperature of the board to at least 450.degree. F. (232.degree. C.) during the heat treating step, as greater stiffness and wet strength are then achieved. This may be because at higher temperatures, shorter step duration is necessary to develop bonding, and there is consequently less time for fiber degradation to occur. Also, shorter required durations enable one to achieve higher production speeds for a treating apparatus of a given length.
While the heat treatment may cover a range of temperatures and durations, these factors are interrelated. Higher temperatures requires a heat treating step of shorter duration, and vice-versa. For example, at 550.degree. F. (289.degree. C.), a duration of 2 seconds has been found sufficient to obtain the desired improvements, while at 420.degree. F., considerably longer is required.
Optionally, the paper may then be subjected to a third step of rewetting the board immediately after the heat treatment and while the paper temperature is above 100.degree. C. to at least 1% mositure by weight. These steps are followed by conventional drying and/or conditioning of the treated board. It is to be understood that steps 2 and 3 can be repeated several times.
Of course, those skilled in the art will recognize the necessity of the product conditioning to a normal moisture content after this very hot treatment. See, for example, U.S. Pat. No. 3,395,219. A certain amount of remoisturizing is normally done, and in fact must be done prior to use or testing. Conventional rehumidification is done after the product has substantially cooled, at temperatures well below 100.degree. C.
Our rewetting treatment differs from conventional conditioning in that we add water, by spraying or otherwise, to a very hot and dry paper or board at the very end of the heat treatment, without intermediate cooling. It is important that the water be applied to the product while it is still hot, certainly above 100.degree. C. (212.degree. F.), and preferably above 205.degree. C. (400.degree. F.). Another heat treatment or drying step may follow rewetting, on or off the machine, during a subsequent operation such as sizing, coating or calendering.